This invention relates to a remote control device for a variable attenuation circuit in a television community receiving system.
A TV amplifier built in a casing is usually provided immediately below a TV wave receiving antenna of a television community receiving system extensively employed today. The amplifier comprises a preamplifier, a main amplifier and a gain control device provided between the preamplifier and the main amplifier, the gain control device being generally controlled by a volume type switch. In most of the amplifiers, the power necessary for operating the amplifier is supplied thereto through a signal transmission coaxial cable from a power supply unit provided in the subscriber's house.
Such conventional community receiving system is shown in FIG. 1, which comprises: a television (TV) wave receiving outdoor antenna 1; an amplifier 2; a power supply unit 3; a television set 4; and a coaxial cable CX.
In FIG. 2, which shows the essential components of the conventional amplifier and the power supply unit, reference character t.sub.1 designates an input terminal for receiving a reception wave from an antenna (not shown), and reference character t.sub.2 designates an output terminal for the amplified reception wave, the output terminal serving also as a power supply input terminal. Between these terminals, a preamplifier 5, a gain control device 6, a main amplifier 7 and a high-pass capacitor C.sub.6 are series-connected as shown in FIG. 2. A choke coil L.sub.11, a diode D.sub.2 with its polarity as indicated and a DC power supply section 8 (including a voltage smoothing circuit and a constant voltage circuit) are connected between a point P.sub.1, between the terminal t.sub.2 and the capacitor C.sub.6, and ground. The DC power supply section 8 feeds current to the preamplifier 5 and the main amplifier 7. These components surrounded by the one-dot chain line l.sub.1 form a community receiving amplifier 2 and are built in a casing except for a knob of the gain control device (not shown), which is provided on the outside of the casing. The amplifier 2 thus formed is provided immediately below the outdoor antenna on the roof. Components surrounded by the one-dot chain line l.sub.2 in FIG. 2 form a power supply unit 3 which is disposed in the subscriber's house or at a position in the vicinity of the house to which the subscriber can readily have access. The power supply unit has terminals t.sub.3 and t.sub.4. The terminal t.sub.3 is connected through a coaxial cable CX to the terminal t.sub.2 of the amplifier 2, and it serves as a terminal for receiving signals from the amplifier and also at terminal for supplying current to the amplifier. The terminal t.sub.4 is to be connected to a television set (not shown). A high-pass capacitor C.sub.7 is connected between the terminals t.sub.3 and t.sub.4. A choke coil L.sub.10 and the secondary coil L.sub.9 of a transformer are connected in series between ground and a point P.sub.2 between the terminal t.sub.3 and the capacitor C.sub.7. The primary coil L.sub.8 of the transformer is connected through a plug to the commercial power supply.
In operation, the 100 V commercial AC power supply voltage is dropped to 20-30 volts by means of the transformer T, which is then applied through the choke coil L.sub.10 and the coaxial cable CX to the amplifier 2, where it is applied through the choke coil L.sub.11 to the diode D.sub.2, as a result of which it is subjected to half-wave rectification and is then applied to the DC power supply section 8. In the DC power supply section 8, the voltage thus rectified is smoothed and is outputted as a constant voltage to operate the preamplifier 5 and the main amplifier 7. Then, a TV wave received by the outdoor antenna is amplified by the preamplifier 5 and main amplifier 7 and then is applied through the coaxial cable CX and the power supply device 2 to the television set.
When it is judged that the level of an input signal to the television set is improper, the level adjustment has to be carried out. In the typical signal level adjustment method, a signal level measuring device is used for measuring an output signal level from the amplifier 2. In this case, someone has to take it to the place where the amplifier 2 is provided, that is, the roof of the house where the outdoor antenna is mounted. At the roof thereof, the measuring device is connected to the output terminal t.sub.2 in order to measure the output signal level from the amplifier 2 and then the output level is adjusted at the proper level by means of the operating knob of the gain control device 6. Furthermore, without the measuring device it is necessary to have the cooperation of many persons. For example, according to the instruction of one person who watches a television, another person who has climbed on the roof operates the knob of the gain control device 6 in order to achieve the adjustment. Both of these two methods are hazardous and troublesome. Further, in the conventional methods, the knob of the gain control device is necessarily protruded from the casing for operation, and this is undesirable with respect to water-proofing.
In addition to the above facts, there is a tendency that the length of the connecting cable becomes longer due to growth of the common antenna television system. This causes a decrease in the output signal level. Therefore, the adjustment of the output signal level of the amplifier must necessarily be frequently carried out.